Electrochemical analysis of aluminium alloy 5182 and 6016 with different surface treatments in 0.1 M NaCl electrolyte
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Replacing steel parts in cars with aluminium (Al) can reduce the weight by up to 50 % and contribute to decreased CO2 emissions. Aluminium is also easily and inexpensively recyclable. A lot of research is therefore conducted in the area of applying aluminium in the automotive industry. High requirements regarding, among others, the corrosion resistance of the materials, are set. Aluminium alloy 5182 and 6016 both meet these criteria. The evaluation of corrosion resistance is, however, complicated due to the complex nature of the corrosion process. The industry therefore needs simplified test procedures that can provide necessary information within a reasonable amount of time. A suitable alternative may be electrochemical techniques. The objective of this thesis was to investigate whether selected polarisation-based electrochemical techniques such as cyclic voltammetry and electrochemical impedance spectroscopy (EIS), including nonlinear EIS, could be used to assess differences in surface treatment of alloy 5182 and 6016, with emphasis on pitting corrosion susceptibility. Samples from three different processing stages were selected for both alloys, namely after annealing, etching and passivation with a conversion coating of titanium (Ti) and zirconium (Zr). The main electrochemical technique used was cyclic voltammetry. The characteristic potentials corrosion potential, Ecorr, pitting potential, Epit, repassivation potential, Erp, and pit transition potential, Eptp, were extracted from the cyclic voltammograms. The electrolyte in all experiments was 0.1 M NaCl. Certain samples subjected to cyclic voltammetry were additionally investigated with stereo microscope and micro X-ray fluorescence (µ-XRF) to study surface appearance and compositional profile as a function of the number of scans performed. The open circuit potential (OCP) of each sample was measured for 3 h. Linear sweep voltammetry was used to estimate polarisation resistance, Rp, after immersion time less than an hour, whereas EIS was used to estimate Rp after 24 h immersion. Capacitance and the nonlinear response of the samples were also recorded with EIS. Amplitudes of both 10 mV and 25 mV were applied in the EIS measurements. A distinct difference between alloy 5182 and 6016 was detected, where samples from alloy 5182 showed a passive region whereas samples from alloy 6016 did not. This was explained through intermetallics and their electrochemical characteristics. XRF analysis indicated that Al and magnesium (Mg) dissolve during the electrochemical testing, thereby enriching the surface with cathodic elements like iron (Fe), silicon (Si) and copper (Cu). The results from electrochemical testing revealed that all samples show susceptibility to pitting corrosion. However, no consistent trend of which sample or surface treatment showing the highest susceptibility or highest resistance to pitting corrosion was found when comparing characteristic potentials obtained from cyclic voltammograms, even though the conversion coating was expected to increase the corrosion resistance. A comparison of the current density of each sample in the cyclic voltammograms, however, showed lower current density for the passivated samples. An examination of the full current distribution on the samples, where local currents from e.g. pitting may also be detected, is recommended due to the limited ability of cyclic voltammetry to detect those kinds of currents. This could reveal more consistent differences between the different surface treatments.